1. Field of the Invention
The present invention relates to a computed tomography (CT) apparatus of the type having a detector system that rotates around a system axis, an image reconstruction computer that is stationary in relation to the detector system, and a data transmission path forth transmission of data from the detector system to the image reconstruction computer, which reconstructs an image from the transmitted data and present this image on a display.
2. Description of the Prior Art
Users of current CT apparatuses as described, for example in Japanese Application 102 29 982, Japanese Application 110 76 226, Japanese Application 112, 35 336 and U.S. Pat. No. 5,867,555, expect a live reconstruction on the monitor, i.e., an image reconstructed and presented on the display (live image) immediately after, or if possible, during, the scanning of a subject under examination using the CT apparatus.
In order to satisfy this expectation, it is necessary that the data supplied by the detector system be transmitted from the rotating detector system to the stationary image reconstruction computer at a data rate that is not lower, or is not significantly lower, than the data rate at which the data are supplied by the detector system.
In particular, in a CT apparatus with a detector system formed by a two-dimensional array of detector elements (known as a multiple-layer CT apparatus), the satisfaction of this requirement encounters difficulties, since the data rate his higher by the same degree as the number of detector elements in relation to a conventional CT apparatus having a one-dimensional array of detector elements (detector row).
An object of the present invention is to provide a CT apparatus of the type described above wherein a live image can be produced even if the data rate at which the data supplied by rotating detector system are transmitted to the stationary image reconstruction computer is lower than the data at which the data are supplied by the detector system.
This object is achieved in a CT apparatus in accordance with the invention, wherein in a first phase during the scanning of the subject under examination (preferably taking place in the form of a spiral scan), data (in the case of a spiral scan, volume data) are extracted, using the data preparation unit, from the data supplied by the detector system and are transmitted to the image reconstruction computer via the data transmission path during the scanning, the computer reconstructs an image (live image) from the extracted data that provides an operator with preliminary information concerning the results of the examination.
The extracted data can, for example, be produced such that, dependent on the algorithm used for the image reconstruction, the data preparation unit forms the mean value from a number of (preferably successive) partial rotation data sets or complete rotation data sets.
In a second phase, after the completion of the scanning of the subject under examination, all data supplied by the detector system during the scanning are transmitted in unmodified form to the image reconstruction computer via the data transmission path.
Since in the first phase only a small part of the data supplied by the detector system during the scanning of the subject under examination need to be transmitted, and the transmission after the scanning of all the data supplied by the detector system during the scanning of the subject under examination is not time-critical, there are no particular requirements with respect to the data transmission rate of the data transmission path. Thus the requirement of rapid production of a first live image can be fulfilled with comparatively low outlay and without the necessity of the data transmission path operating at a high data rate. Depending on the way in which the extracted data are obtained, for the transmission of the extracted data a data rate is sufficient that is on the order of magnitude of one-fourth of the data rate that would be required for the transmission of all the data during the scanning.
The invention is in particular suitable for multiple-layer CT apparatuses whose detector system is a two-dimensional array of detector elements that are preferably arranged in a number of rows and columns. However, the invention is not limited to CT apparatuses of this sort, but is suitable for CT apparatuses whose detector system is a one-dimensional array of detector elements, such as a detector row.
Moreover, the invention is in particular suitable for CT apparatuses known as spiral CT apparatuses, in which during a scanning a continuous rotation of the detector system about the system axis takes place, with simultaneous displacement of the subject under examination and the detector system relative to one another, in the direction of the system axis. In a CT apparatus of this type, considerable quantities of data can occur during a scanning, whose transmission during the scanning would be problematic without the invention.